Boys show off their catch on the shoreline of Lake Malawi near Salima in 2012. About the size of New Jersey, Lake Malawi is one of the most biologically diverse lakes in the world.

Ding Haitao
/ Xinhua/Landov

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Originally published on May 28, 2014 4:17 pm

People trying to grow food and support their families on the shores of Lake Malawi are not only causing serious environmental problems, they're also causing a surge in a debilitating disease.

Thriving towns along the lake are changing the ecosystem in ways that are allowing a parasitic worm to flourish, researchers reported last week in the journal Trends in Parasitology.

The intestinal disease, called schistosomiasis, won't kill you, but it can make you pretty sick with a fever, cough and muscle aches. Schistosomiasis comes from flatworm parasites that burrow into people's skin usually when they're bathing in warm, shallow water. Kids who get infected repeatedly can develop malnutrition and learning difficulties.

"In some villages around Lake Malawi up to 70 percent of the people and 95 percent of schoolchildren are infected [with schistosomiasis]", says Bert Van Bocxlaer, a postdoctoral researcher with the Smithsonian Institution and Ghent University in Belgium.

About the size of New Jersey, Lake Malawi is a huge source of water, electricity and food for people in Tanzania, Mozambique and Malawi. Over the past 30 years, the number of people living around the lake has more than doubled to about 14 million. Agricultural output has risen dramatically.

Many things that people have done to try to make their own lives better have made the schistosomiasis situation worse, Van Bocxlaer and his colleagues found. Migration, intense fishing, and planting larger and larger tracts of land have all led to the proliferation of the parasitic worm that causes schistosomiasis.

The worm's life cycle takes it back and forth between living in snails and in people. When a person is in the water, the worms penetrate the skin and then mate inside the intestines. Human excrement then carries the parasite eggs back into the lake where they hatch and go to live inside a species of freshwater snails called Bulinus nyassanus in Lake Malawi. Once the worms are large enough, the parasites swim away from the snails and go out in search of the next human host.

In the past, large schools of fish have kept the snail populations in Lake Malawi in check. But Van Bocxlaer says that's changed. "There has been overfishing of the fish that usually eat these snails," he says.

In addition, increased farming around the lake has caused sediment levels to rise. This has made the aquatic environment even more favorable for the problematic snails. More snails mean more parasites and more disease.

"People are doing more agriculture and more fishing because they need food," Van Bocxlaer says. "But on the other hand, intensified use of these natural resources really causes effects in the aquatic ecosystem that increase the likelihood of transmission of this disease."

Van Bocxlaer himself came down with schistosomiasis while doing this research. Being sick made him think even more deeply about the problem. Understanding the ways in which humans are contributing to the increase in the disease, he says, is an important step in figuring out eventually how to stop it.

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Transcript

ROBERT SIEGEL, HOST:

It's ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

MELISSA BLOCK, HOST:

And I'm Melissa Block. Now we're going to hear how a rise in a debilitating disease is being fueled in part by people trying to improve their lives.

In Central Africa, a boom in agriculture and fishing on the shores of Lake Malawi is changing the environment and in the process creating more favorable conditions for a parasitic worm. NPR's Jason Beaubien reports on research that drew the link.

JASON BEAUBIEN, BYLINE: Schistosomiasis usually won't kill you, but it can make you pretty sick. The intestinal disease is also known as bilharzia. It comes from flatworm parasites that burrow into people's skin, usually when they're bathing in warm, shallow water. In many lakes in Central Africa, including Lake Malawi, it's a big problem.

BERT VAN BOCXLAER: In some villages around Lake Malawi, up to around 70 percent of the people and 95 percent of schools' children are infected.

BEAUBIEN: Bert Van Bocxlaer is a postdoctoral researcher with the Smithsonian and Ghent University in Belgium. Most of his research is on the evolutionary patterns of snails. Van Bocxlaer was studying snails in Lake Malawi, including the snails that carry schistosomiasis, when he realized that the increase in disease was being driven by people. Over the last 30 years, the number of people living around the lake has more than doubled. Agricultural output has risen dramatically. But Van Bocxlaer found that many of the things that people have done to try to make their own lives better have also made things better for the parasites and worse for human health.

BOCXLAER: There has been overfishing of fish that usually eat these snails.

BEAUBIEN: Additional farming on the shores of the lake has pushed more sediment into the water, which has improved the habitat for the problematic snails.

BOCXLAER: The second aspect - there has been another snail that has been introduced from Asia that is occurring there now and that may be favored by the fish that, before, ate the intermediate host of this disease.

BEAUBIEN: The parasite cycles between the snails and people. In the water, the tiny worms penetrate people's skin. The parasites mate inside people's intestines. Human excrement then carries the parasite's eggs back into the lake, where they hatch and go on to live inside the snails.

Once they're large enough, they swim away from the snails and go out in search of another human host. This cycle has gone into overdrive recently, as the number of predatory fish has gone down and the number of people in the area has gone up.

BOCXLAER: People are doing more agriculture, more fishing because they need food, right? But on the other hand, intensified use of these natural resources really causes an effect in the aquatic ecosystem that increased the likelihood of transmission of this disease.

BEAUBIEN: Then Bocxlaer himself came down with schistosomiasis while doing this research, which has been just published in Trends In Parasitology. He says, this made him think even more deeply about the problem. He says, understanding the ways in which humans are helping to fuel the disease is an important step in figuring out, eventually, how to stop it. Jason Beaubien, NPR News. Transcript provided by NPR, Copyright NPR.